In relatively quick succession two family cars decided to have flat batteries namely the MK 1 and the Datsun 120Y. There was no basic reason for this problem as both vehicles had relatively new batteries. Both were recharged and the MK 1 had the battery lead removed from the battery terminal.
About two weeks later I went to start the Datsun but only got a very weak starter and no inclination to fire. I promptly hit it with jumper leads and it started and run normally. However this loss of electrical power was a problem that needed solving. I connected the multimeter on amps range between the battery and the lead and noted a very slight drain of under 1/10 of an amp. In fact it was about 80 Ma. Now that is not a high discharge as 80 Ma equals 80 thousandths of an AMP.
However there are 168 hours in a week and 80Ma would mean a loss of electrical storage of about 13-ampere hours per week. The battery in the Datsun is a 7 plate 260 CCA which is more than adequate for the task but the overall capacity is about 35 ampere hours i.e. 1 amp discharge at 12 volts for 35 hours. What that meant that even a fully charged battery would go flat in approximately 21/2 weeks.
What was causing it was very simple. The electric clock was the electricity thief. I pulled the clock fuse in the Datsun, which also disabled the cigarette lighter. There was no other drain on the battery and the subsequent discharge on the multimeter was zero.
Heartened by this discovery I attacked the MK 1 and found out that the clock was also the culprit in this car. Now I had been very successful in even getting a MK 1 clock to go and was rather proud of it. But there is little doubt that these devices are power hungry. So I resorted to fitting a quick disconnect to the battery post so that I can easily disconnect the electrical power drains prior to the car going into a typical couple of weeks of storage between being used.
There is another power drain commonly missed in the more modern jags fitted with alternators. This is the residual or diode bias flow which occurs in modern alternator electrical system which use diodes rather than cut-out switches. Note - no matter how perfect a diode is it will always have electrical leakage. This means that if you park your XJ6 for a month or two there is a chance of the battery going flat.
